1. Field of the Invention
The invention relates to a pump control apparatus that drives a pump using a motor.
2. Description of the Related Art
Some known brake control systems control the braking force of a vehicle by generating hydraulic pressure in a hydraulic circuit according to the operating force applied to a brake pedal and supplying that hydraulic pressure in the hydraulic circuit to a wheel cylinder. One such brake control system brakes a vehicle by supplying hydraulic pressure, which is different than a master cylinder pressure, from an accumulator serving as a hydraulic pressure source to the wheel cylinder. This kind of a system controls the hydraulic pressure by detecting the operating amount of the brake pedal by the driver and supplying brake fluid from the accumulator to match the wheel cylinder pressure corresponding to the operating amount.
Accumulation in the accumulator is achieved by driving a pump. However, if the amount of brake fluid in the hydraulic circuit falls below a specified amount, brake fluid cannot be discharged normally from the pump. As a result, sufficient accumulation in the accumulator is not possible so brake fluid is unable to be appropriately supplied from the accumulator to the wheel cylinder, making high-precision braking difficult. Therefore, if such a state is detected in the brake control system, various control valves are controlled to close off the flow path from the accumulator to the wheel cylinder and supply brake fluid to the wheel cylinder from another flow path. The various methods for detecting a decrease in brake fluid have been proposed, such as monitoring the level of brake fluid in a reservoir tank.
The current supplied to a motor for driving the pump changes depending on the load. Therefore, Japanese Patent Application Publication No. 2005-96613 (JP-A-2005-96613) describes a pump-driving motor control apparatus that estimates the pump load by monitoring a change in the output voltage between the motor for driving the pump and the power supply.
However, with the control apparatus described in JP-A-2005-96613, the output voltage is always lower than the power supply voltage so the difference in the output voltage when the pump load is high versus when the pump load is low is not that great, which limits the estimation accuracy of the pump load.